Sheet processing apparatuses so-called finishers that perform post-processing such as aligning, sorting, stapling, and/or punching of sheets are widely used. Sheet processing apparatuses typically include a stapling tray, on which sheets, for example, discharged from the image forming apparatus, are stacked, an alignment unit to align the sheets on the stapling tray in the direction (i.e., sheet width direction) perpendicular to the direction in which the sheets are transported (hereinafter “sheet conveyance direction”), a reference fence to align the sheets in the sheet conveyance direction, and a stapler that staples the aligned sheets. The stapler is typically movable in the sheet width direction along a side (typically, the trailing side) of a bundle of sheets that is in contact with the reference fence for stapling an end portion of the bundle (i.e., side stapling). Thus, the stapler is moved for stapling multiple positions of the bundle.
The length of the stapled side of the bundle of sheets differs depending on sheet size or direction of sheets. Accordingly, if the stapler staples sheets at the same positions (hereinafter “stapled positions”) regardless of sheet size or direction of sheets in two-position stapling, the interval betweens the two positions stapled is greater in the case of small sheets.
To adjust the interval betweens the stapled positions in two-position stapling, the stapler may be designed to be set at given positions for stapling. This approach, however, has a drawback in that it is possible that the reference fence blocks the stapler moving to an intended position to be stapled and stapling cannot be performed.
In view of the foregoing, for example, in JP-2009-242014-A, the reference fence is designed to move in the sheet width direction to a position not to interfere with the stapler after the sheets are aligned. However, moving the reference fence after alignment of the sheets can disturb alignment of the sheets. Additionally, the position to which the reference fence is moved may be improper for supporting the sheets, making the sheets unbalanced. Thus, it is possible that alignment of the sheets is degraded. Moreover, moving the reference fence after alignment of the sheets causes a downtime in processing, reducing productivity.